Influence of fire on root distribution, seasonal root production and root/shoot ratios in grass species in a semi-arid grassland of South Africa

The impact of fire (head and back fires) on the seasonal change in above- and belowground production in grass species, was quantified over two growing seasons (2000/01 and 2001/02) for semi-arid grassland. The behaviour of the head and back fires was also identified. Roots were extracted to a depth of 900 mm with 50 mm intervals, using a core and then separated from the soil by wet sieving and flotation. Sampling of both above- and below-ground phytomass, took place at approximately bi-monthly intervals to account for major seasonal changes. The intensity of the back fire was higher than the head fire at ground level. Most of the grass roots were found in the first 150 mm soil layer. While fire increased grass root distribution over the first 100 mm depth, it was decreased deeper in the soil. Root mass in semi-arid grassland is strongly seasonal, with the most active growth during the months of March and April when aboveground parts are dormant. Both above- and belowground phytomass production decreased significantly over the first year following burning. The above- and belowground phytomass (900 mm depth) was respectively 806 kg ha− 1 and 2 002 kg ha− 1 less due to burning, over the first year after burning. The seasonal root/shoot ratios for the unburnt and burnt grassland ranged between 1.62 to 2.80 and 1.20 to 3.12 respectively. It seems that root mass exceeds aboveground biomass for this semi-arid grassland. Over the short-term, fire decreased productivity and could subsequently influence the sustainable fodder production of a semi-arid grassland ecosystem.